The invention relates to a combined current and voltage measuring apparatus with a cast resin body which supports the current measuring unit and is also utilized as dielectric for the capacitor on the high-voltage side of the capacitive voltage divider equipped with an amplifier of a capacitive measuring unit.
A combined current and voltage transformer is known from Deutsche Auslegeschrift No. 1,114,921 wherein the voltage transformer unit contains a capacitive divider. The dielectric of the winding insulation of the current transformer unit is also utilized as dielectric for the capacitor on the high-voltage side of the capacitive divider in that there are applied to the winding insulation conducting coatings forming the electrodes of the high-voltage capacitor.
In the combined current and voltage transformer of the known construction, inaccuracies of the voltage measurement may result because the divider ratio determined by the selection of the high-voltage and the low-voltage capacitors can change because of different temperature changes in these respective capacitors. The cause for different warming of the high-voltage and the low-voltage capacitors can be attributed to the condition that the primary conductor of the current transformer unit, heated by the current flowing through it, heats up and transfers its heat to the adjacent winding insulation which is utilized at the same time as dielectric of the high-voltage capacitor; this results in a variation of the capacitance of the high-voltage capacitor and, hence, to a variation of the divider ratio of the capacitive voltage which, in turn, leads to measuring inaccuracies.
Accordingly, it is an object of the invention to provide a combination current and voltage measuring apparatus wherein a cast resin body serving as dielectric for the high-voltage capacitor is configured in such a manner that a warming of the high-voltage capacitor different from the warming of the low-voltage capacitor does not influence the measuring accuracy of the capacitive voltage measuring unit of the apparatus.